Energy efficient transmission and reception are becoming increasingly important within the field of wireless communications. The main reason for this is the expected massive growth of low cost devices for Internet of Things, IoT.
Energy efficiency can mean two different things. On the one hand, it may refer to the transmitter side and that one should be able to transmit the information using as low power as possible, either just to save energy or because there may be regulatory limitations for what is allowed. On the other hand, it may refer to the energy needed to receive the signal. Typically, one can trade the power consumption in the transmitter and the receiver in the sense that it is often possible to achieve reliable communications at lower transmission power by allowing a higher complexity and power consumption in the receiver. The higher complexity typically implies that efficient forward error correcting codes are used, such as e.g. convolutional codes, low density parity check codes, or so-called Turbo codes.
One approach for achieving low power consumption in a device is to use so-called Wake-Up Receivers, WURs, sometimes also referred to as wake-up radios, in combination with a Primary Radio, or a primary receiver. These WURs provide a means to significantly reduce the power consumption in receivers used in wireless communication. The idea with a WUR is that it can be based on a very relaxed architecture, as it only needs to be able to detect the presence of a wake-up signal, but will not be used for any data reception. The Primary receiver is then arranged for data reception.
A commonly used modulation for the Wake-Up Signal, WUS, i.e., the signal sent to the WUR, is On-Off Keying, OOK. OOK is a binary modulation, where a logical one is represented with sending a signal, ON, whereas a logical zero is represented by not sending a signal, OFF.
There are currently activities ongoing in the Institute of Electrical and Electronics Engineers, IEEE, 802.11 task group named IEEE 802.11ba to standardize the Physical layer, PHY, and the Link Layer, MAC, for a Wake-Up Radio to be used as a companion radio to the main 802.11 radio with the mere purpose to significantly reduce the power consumption.
It is proposed to generate the WUS by using an Inverse Fast Fourier transform, IFFT, as this block is already available in Wi-Fi transmitters supporting 802.11a/g/n/ac, for example. Specifically, an approach discussed for generating the OOK is to use the 13 sub-carriers in the center, and then populating these with some signal to represent ON and to not transmit anything at all to represent OFF. The IFFT has 64 points and is operating at a sampling rate of 20 MHz, and just as for ordinary Orthogonal Frequency Division Multiplexing, OFDM, a cyclic prefix is added after the IFFT operation in order to keep the OFDM symbol duration used in 802.11a/g/n/ac and thus be able to spoof legacy stations by prepending a legacy preamble at the beginning of the WUS. In this way, legacy stations will be able to detect the WUS and correctly defer access to the wireless medium.
The design of the WUS as described above allows for a very power efficient receiver implementation. However, in order to achieve this, the bandwidth of the signal is limited to about 4 MHz.
To achieve low power consumption in the WUR, it is desirable to use a low sampling rate in the receiver, which translated to having a channel selective filter with a relatively small bandwidth. On the other hand, due to regulatory requirements, the maximum transmission power that can be used is often limited by the Power Spectrum Density, PSD. That is, although a higher total transmission power would be allowed, this cannot be used if the bandwidth of the signal is too small. Since a reduced transmission power directly means that the range of the WUR will be reduced, this results in a trade-off between receiver power consumption and transmission range.